The World of Plants

a little database

The Cell

Plant Cell

Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:

  • A large central vacuole, a water-filled volume enclosed by a membrane known as the tonoplast maintains the cell’s turgor, controls movement of molecules between the cytosol and sap, stores useful material and digests waste proteins and organelles.
  • A cell wall composed of cellulose and hemicellulose, pectin and in many cases lignin, are secreted by the protoplast on the outside of the cell membrane. This contrasts with the cell walls of fungi (which are made of chitin), and of bacteria, which are made of peptidoglycan.
  • Specialised cell-cell communication pathways known as plasmodesmata, pores in the primary cell wall through which the plasmalemma and endoplasmic reticulum of adjacent cells are continuous.
  • Plastids, notably the chloroplasts which contain chlorophyll and the biochemical systems for light harvesting and photosynthesis, but also amyloplasts specialized for starch storage, elaioplasts specialized for fat storage and chromoplasts specialized for synthesis and storage of pigments. As in mitochondria, which have a genome encoding 37 genes plastids have their own genomes of about 100-120 unique genes and probably arose as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.
  • Unlike animal cells, plant cells are stationary.
  • Cell division by construction of a phragmoplast as a template for building a cell plate late in cytokinesis is characteristic of land plants and a few groups of algae, notably the Charophytes and the Order Trentepohliales
  • The sperm of bryophytes have flagellae similar to those in animals, but higher plants, (including Gymnosperms and flowering plants) lack the flagellae and centrioles that are present in animal cells.

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What else is there? Here are the three most important components of the Plant cells (and of ALL eukaryotic cells!)

Mitochondria: membrane-bound organelle found in the cytoplasm of almost all eukaryotic cells (cells with clearly defined nuclei), the primary function of which is to generate large quantities of energy in the form of adenosine triphosphate (ATP).
Mitochondria are typically round to oval in shape and range in size from 0.5 to10 μm. In addition to producing energy, mitochondria store calcium for cell signaling activities, generate heat, and mediate cell growth and death.

Golgi Apparati: membrane-bound organelle of eukaryotic cells (cells with clearly defined nuclei) that is made up of a series of flattened, stacked pouches called cisternae.
The Golgi apparatus is responsible for transporting, modifying, and packaging proteins and lipids into vesicles for delivery to targeted destinations. It is located in the cytoplasm next to the endoplasmic reticulum and near the cell nucleus.

And, last but not least, the Nucleus: it controls and regulates the activities of the cell (e.g., growth and metabolism) and carries the genes, structures that contain the hereditary information (D.N.A.)

 

 

December 26, 2010 Posted by | Plant cell, PLANT PHYSIOLOGY | Leave a comment

Secondary thickening- Lignification

 

In many vascular plants, secondary growth is the result of the activity of the vascular cambium, a meristem that divides to produce secondary xylem cells on the inside of the meristem (the adaxial side) and secondary phloem cells on the outside (the abaxial side).
This growth increases the girth of the plant root or stem, rather than its length, hence the phrase “secondary thickening”. As long as the vascular cambium continues to produce new cells, the stem or root will continue to grow in diameter. In woody plants, this process produces wood.

December 26, 2010 Posted by | PLANT PHYSIOLOGY, Secondary thickening - Lignification | Leave a comment

Phloem

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), particularly sucrose, a sugar, to all parts of the plant where needed.

The main conducting elements of the phloem are the sieve elements, of which there are two different types: sieve cells and sieve-tube members.
Sieve cells have narrow pores, their sieve areas are quite uniform in structure, and they are distributed evenly on all walls. One of the principal differences between sieve cells and sieve-tube members is the presence of sieve plates in sieve-tube members, that are absent in sieve cells.
Sieve cells are the only type of food-conducting cells in most seedless vascular plants and gymnosperms, whereas in angiosperms only sieve-tube members are present. Sieve-tube members occur end-on-end in longitudinal series called sieve tubes. They are in contact via plasmodesmata.
Typically, the final walls are interspersed with primary pit areas (groups of plasmodesmata), that later on develop into sieve plates. Sieve tubes in the phloem of angiosperms are flanked by one or several plasma-rich, nucleated companion cells, that do not occur in gymnosperms.

 

December 26, 2010 Posted by | Phloem, PLANT PHYSIOLOGY | Leave a comment

Xylem

Xylem and phloem

The xylem is part of the vascular system that conveys water and dissolved minerals from the roots to the rest of the plant and may also furnish mechanical support.
It consists of specialized water-conducting tissues made up mostly of narrow, elongated, hollow cells. These cells may be of several types, including tracheids (the basic cell type), vessel members, fibres, and parenchyma.
Xylem constitutes the major part of a mature woody stem or root: the wood of a tree is composed of xylem. (see “Secondary thickening – Lignification” post for further details)
Xylem formation begins when the actively dividing cells of growing root and shoot tips (apical meristems) give rise to primary xylem. As the growing part of the plant builds past the xylem thus formed, the vascular cambium produces secondary xylem tissues that cover the primary xylem.
When this happens the primary xylem cells become dead and empty, losing their conducting function and forming a hard skeleton that serves only to support the plant. Thus, in the trunk and older branches of a large tree only the outer part of the wood (secondary xylem) serves in water conduction, while the inner part (heartwood) is composed of dead but structurally strong primary xylem.

December 24, 2010 Posted by | PLANT PHYSIOLOGY, Xylem | Leave a comment